1. Field of the Invention
The present invention relates generally to memory systems, and more particularly, to methods, systems and apparatus for programming phase change memory cells.
2. Description of the Related Art
The general concept of utilizing electrically writable and erasable phase change materials (i.e., materials that can be electrically switched between generally amorphous and generally crystalline states) for electronic memory applications is well known in the art. A typical phase change material is a material that has two general states: a generally amorphous state and a generally crystalline state. The phase change material can include one or more chalcogenide compounds that at least partly include one or more of the following materials: Te, Se, Sb, Ni, and Ge and various combinations thereof.
The typical phase change material can be switched from one state to the other by passing an electrical current or other type of energy through the phase change material to cause it to change states. Typically in the first state (e.g., amorphous state), the phase change material has a relatively high resistance and in the second state (e.g., the crystalline state), the phase change material has a relatively low resistance. The resistance ratio between amorphous state and crystalline state is about 1000:1.
As the state of the phase change material can only be changed by a sufficient application of energy (e.g., a programming energy pulse), then the phase change material is generally non-volatile in that it does not require energy to maintain its current state. Further, because the resistance of the phase change material varies with the state (e.g., a low resistance for a crystalline state and a high resistance for an amorphous state), then the phase change material can be reliably used to store binary data such as may be used for a memory cell in a computer or other binary data storage usage.
The programming energy pulse determines the actual resistance of a programmed phase change device. By way of example, a first programming energy pulse (i.e., 1 ms pulse of 1.42 mA) is applied to a phase change device and results in a resistance of 50 ohms. If a second programming energy pulse (i.e., 1 ms pulse of 1.98 mA) applied to the same phase change device a resistance of 500 ohms could result. As a result only very slight variations in the amount of energy (e.g., electrical current in this instance) results in markedly different resistance levels of the phase change device. Further, as the process variations (e.g., film thickness of the phase change material) and the operating parameters (e.g., operating temperature, voltage, etc.) vary, then the energy required to achieve the desired resistance level changes. Therefore, it is difficult to accurately program a phase change device to a selected resistance level.
In view of the foregoing, there is a need for a system and method for accurately and quickly programming multiple data values in a phase change device.